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CHEMISTRY · Stage 7 · Chapter 6: The Earth and Plate Tectonics

Chemistry 6.2 Plate tectonics — Tectonic Plates and Mantle Convection

5 min read 7 parts Lesson 2 of 6

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6.2 Plate tectonics — Tectonic Plates and Mantle Convection — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe the model of plate tectonics, the evidence for it and how convection in the mantle moves the plates.Wegener's observation — in the early 1900s Alf…

  • Hypothesis (1912)
  • More evidence
  • Convection current
  • Other forces
  • Measurement
  • Atlantic Ocean

Part 1 of 7A hypothesis

  • Wegener's observation — in the early 1900s Alfred Wegener saw that Africa and South America look as if they fit together and wondered if they had once been joined.
  • Evidence he found:
    • Similar rock types at the edges of the continents where they might have been joined.
    • Fossils of the same reptile, Mesosaurus, in Africa and South America, which lived in shallow water and could not swim across the ocean.
  • Hypothesis (1912) — the continents were once a single piece of land, Pangaea, which broke up as the pieces drifted apart.

Part 2 of 7Rejection and acceptance

  • Rejection — other scientists did not know how continents could drift apart and did not trust Wegener because he mainly studied the weather, not rocks.
  • More evidence — different scientists found more evidence and developed the idea, and by 1964 many scientists accepted the model of plate tectonics.
    • Coastlines of South America and Africa fit together like jigsaw pieces.
    • Matching rock layers and mountain ranges are found across oceans.
    • Sea-floor spreading: new rock forms at mid-ocean ridges, magnetic stripes match on both sides, and rock gets older away from the ridge.
    • Earthquakes and volcanoes mark the plate boundaries.

Part 3 of 7The model of plate tectonics

  • Tectonic plates — the Earth's crust and the uppermost mantle (the lithosphere) are made of about 12 huge slabs of solid rock.
    • The plates rest on the lower part of the mantle.
  • Movement — heat from deep inside the Earth makes the lower mantle flow, even though it is solid, and the plates move with the flowing mantle at a few centimetres a year.
  • Explains — continental drift, earthquakes, volcanoes and mountains.

Part 4 of 7Mantle convection

  • Heating — the mantle is heated from below by the hot core and by radioactive decay.
  • Convection current — hot rock expands, its particles move apart, it becomes less dense and rises very slowly.
    • Near the top it cools, becomes denser and sinks.
    • The circular movement drags the plates along.
  • Other forces — slab pull is the weight of a cold sinking plate pulling the rest behind it, and ridge push is new rock at the ridge pushing the plate away.

Part 5 of 7Plate boundaries

BoundaryMovementResults
DivergentApartNew crust, ridges, rift valleys
ConvergentTogetherTrenches, volcanoes, fold mountains
TransformSliding pastEarthquakes

Part 6 of 7Rates of movement

  • Measurement — satellites using GPS measure plate movement.
  • Atlantic Ocean — it is widening by about 2.5 cm a year while the Pacific is slowly shrinking.
    • At 2.5 cm a year a width of about 5000 km (500 000 000 cm) takes 500 000 000 ÷ 2.5 = 200 000 000 years.
    • The result agrees with the age of the oldest ocean floor and the time since Pangaea broke up.

Part 7 of 7How scientists build on each other's ideas

  • Collaboration — most scientists do not work in isolation but build on the work of earlier scientists and work jointly with others.
    • Wegener used evidence from other scientists, and others built on his ideas.
  • Sharing — they talk and message each other, give talks at conferences or webinars, show posters and write papers for journals.
  • Peer review = other expert scientists checking a paper, which is published in a scientific journal if it is good enough.

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